I love my 22" - I'm going on 5 years with it, and I only have one seam rip. (from when it was thrown in the back of the DC-9 by our MX people) Everyone has said all of the basics... it really is a great bag. You will keep finding pockets in it that you didn't know it had. I literally just discovered a NEW zipper pocket about two weeks ago.
Let's see... on the CRJ (Not the ERJ - I always had to put the bag on the cart when DHing on the ERJ) I would always have enough room for: Toiletry Kit, Tennis Shoes (or whatever you kids are calling them these days... cross trainers, gym shoes??? I don't know), 4-6 days worth of clothes - including uniforms. I would put my macbook in a sleve and put in in the front/bottom of the bag. (I never gave my bag to the rampers - I would be able to slide it in in both the -200 and -700 overhead bins.)
For commuting - I would recommend the 22" - fits perfectly in every overhead bin out there - except the first couple of rows on aircraft left of the older SWA 73's - and the 767. (which is a odd duck when it comes to overhead space in the first place - but domestically, there is always a cubbyhole that the FA's can shove your bag in if you are the last one on.) The 26" will always have to be placed sideways on the commute. (But, will fit sideways very well in the CRJ's)
Get the custom strap - my favorite one that I saw was "Nacho Bag" in orange. I just have my name. Also, it has a couple of good places for stickers - I have an In 'n Out, an Apple, and a couple of random ones on the bottom. No trouble identifying my own bag in a lineup.
I would recommend the metal frame one - even though it is heavy, I think that it holds up to the rigors of day to day flying. (5-8 legs - rain, sleet, snow) On the Bus these days, I've noticed that my bag doesn't take much of a beating.
I've replaced my wheels twice... by myself - each time, no problem.
